The prior art has prepared filaments from polymeric compositions such as polyacrylonitrile by the conventional technique of melt spinning into fibers or filaments which can be converted into multi-filament assemblies and thereafter oxidatively stabilized. Such fibers or assemblies are then subjected to carbonizing procedures to improve fire resistance.
Expanded fibers are desirable because they provide excellent feeling, bulkiness and elasticity. Crimped expanded fibers are even more desirable because the bulkiness is increased together with rapid return after compression. Such fibers find particular use as insulation for clothing, carpet material and in fiber blends for fabric.
Attempts have been made to prepare crimped aromatic fibers. U.S. Pat. No. 4,120,914, discloses the preparation of highly crimped fibers of poly(p-phenylene terephthalamide) which as a result of the mechanical crimping suffers from mechanical damages that often results in an appreciable decrease in fiber tenacity. The crimping is performed by a steam stuffer-box crimping process which produces bending strains in the fibers.
Stuffer box crimping results in sharp V-type bends in the fiber that produces excessive tension on the outer bend and severe compression on the underside. This leads to unacceptable fiber damage especially with rigid or stiff fibers.
In the Paper of Hall et al entitled "Effects of Excessive Crimp on the Textile Strength and Compressive Properties of Polyester Fibers", J. of Applied Polymer Sci, Vol. 15, p. 1539-1544 (1971), there is described the effect of forming sharp crimps on polyester fibers as well as other man-made fibers. Excessive crimping such as found in the V-type crimps leads to surface damage of the fiber and a reduction in tenacity and elongation properties.
U.S. Pat. No. 4,837,076, to Mc Cullough, Jr. et al, which is herein incorporated by reference, relates to the preparation of non-linear carbonaceous fibers and to carbonaceous fibers having different electroconductivity. This patent discloses a process which can be used to heat treat and carbonize expanded polymeric fibers to yield the fibers of the invention.
U.S. Pat. No. 4,752,514, to Windley, which is herein incorporated by reference, discloses crimped and expanded polyamide fibers. The crimps in the fiber are caused by collapsed portions. There is also disclosed a process for preparing the precursor fibers useful in the present invention.
U.S. Pat. No. 4,788,093, to Murata et al, which is herein incorporated by reference, discloses porous expanded acrylonitrile based fibers and a process for their preparation. The process can be used for preparing one of the precursor fibers of the invention.
U.S. Pat. No. 4,832,881, to Arnold Jr. et al, discloses the preparation of low density, microcellular carbon foams from polyamides, cellulose polymers, polyacrylonitrile, etc. The foams are rigid and brittle.
U.S. Pat. No. 4,193,252, to Sheppherd, et al discloses the making of partially carbonized, graphitic and carbon fibers from stabilized rayon which have been knitted into a carbon assembly. When the fabric is deknitted, the partially carbonized and the carbonized fibers contain kinks. The fully carbonized or graphite fibers have kinks which are more permanent in nature. It has now been found that partially carbonized rayon fibers are flammable, do not retain their reversible deflection and lose their kinks at relatively low temperatures or under tension.
U.S. Pat. No. 4,642,664, of Goldberg et al, which is herewith incorporated by reference, discloses the use of carbonized aromatic polyamides for use as conductors in electrical devices. However, there is only disclosed non-expanded fibers.
It is understood that the term "expanded fiber" as used herein includes porous, hollow or cellular fibers, or a combination thereof.
All percentages herein are by weight unless otherwise indicated.
The carbonaceous expanded fibers of the invention have a limited oxygen index value greater than 40, as determined by test method ASTM D 2863-77. The test method is also known as "oxygen index" or "limited oxygen index" (LOI). With this procedure the concentration of oxygen in O.sub.2 /N.sub.2 mixtures is determined at which a vertically mounted specimen is ignited at its upper end and just continues to burn. The size of the specimen is 0.65.times.0.3 cm with a length of from 7 to 15 cm. The LOI value is calculated according to the equation: ##EQU1##
The term "stabilized" herein applies to fibers or tows which have been oxidized at a specific temperature, typically less than about 250.degree. C. for acrylic fibers. It will be understood that in some instances the filament and/or fibers are oxidized by chemical oxidants at lower temperatures.
The term "reversible deflection" as used herein applies to a helical or sinusoidal compression spring. Particular reference is made to the publication, "Mechanical Design--Theory and Practice," MacMillan Publishing Co., 1975, pp 719 to 748, particularly Section 14-2, pp 721 to 724.
The term "carbonaceous fiber" relates to polymeric fibers whose carbon content has been irreversibly increased as a result of a chemical reaction such as a heat treatment as disclosed in U.S. Pat. No. 4,837,076, and is at least 65%.
The term "fibrous structure" as utilized herein is intended to mean an arrangement of one or more fibrous elements or materials into a complex entity such as a textile fabric which includes mats, battings, knitted, woven and non-woven materials, and the like.
The term "non-graphitic" relates to those carbonaceous fibers having an elemental carbon content of not more than 92%, are substantially free of oriented carbon or graphite microcrystals of a three dimensional order, and are as further defined in U.S. Pat. No. 4,005,183, which is herein incorporated by reference.